Composite materials with tailored electromagnetic spectral properties, structural elements for enhanced thermal management, and methods for manufacturing thereof

ABSTRACT

Disclosed is a method to produce composite materials, which contain customized mixes of nano- and/or micro-particles with tailored electromagnetic spectral properties, structural elements based thereon, in particular layers, but also bulk materials including inhomogeneous bulk materials. In some embodiments the IR-reflectivity is enhanced predominantly independently of reflectivity for visible wavelength. The enhanced IR-reflectivity is achieved by combining spectral properties from a plurality of nano- and/or micro-particles of distinct size distribution, shape distribution, chemical composition, crystal structure, and crystallinity distribution. This enables to approximate desired target spectra better than know solutions, which comprise only a single type of particles and/or an uncontrolled natural size distribution. Furthermore disclosed are methods of manufacturing such materials, including ceramics, clay, and concrete, as well as applications related to design and construction of buildings or other confined spaces.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to methods of creating composite materials withtailored spectral properties in the visible and invisibleelectromagnetic (EM) wave spectrum using a plurality of discretepopulations of nano- and/or micro particles and/or nano- and/or microcavities.

In some embodiments the invention relates to structural elements, whichenable the design and construction of buildings or other confined spaceswith improved thermal and energetic properties. In particular, theinvention enables the construction of buildings, which absorb less solarradiation per given surface area. This results in lower temperaturesinside buildings or other confined spaces, increased time to reachhigher temperatures inside such buildings or predominantly confinedspaces, and/or in reduced power requirements for air-conditioningsystems.

Description of the Related Art

Many technical applications require to control the temperature insidepredominantly enclosed spaces or, as a related aspect, the control theelectromagnetic spectral properties of materials and surfaces. In someinstances it may be desirable to keep the temperature insidepredominantly enclosed spaces low, or even as low as possible, whereasin other applications the opposite may be the case. In yet otherinstances, for example related to achieving broadband camouflageeffects, the electromagnetic spectral “appearance” may be of primaryinterest.

The broadband electromagnetic spectral properties of surface materials(within a certain wavelength range) can affect:

-   -   (a) how a material/surface visually appears,    -   (b) how readily visible and invisible electromagnetic radiation        is absorbed from a surface and thus gets converted into thermal        energy, and    -   (c) how rapidly a surface can lose thermal energy by emitting        electromagnetic waves at thermal wavelength.

The invention relates in particular to the design and manufacturing ofcomposite materials, which have tailored and improved Ultraviolet (UV),visible (VIS), Near Infrared (NIR), Mid Infrared (MIR), and Far Infrared(FIR) properties, by containing tailored mixtures of nano- and microparticles. Henceforth the term ‘light’ shall be understood to apply forall of these spectral ranges, not only the portion visible to humans.

Embodiments herein may be described within the context of a range ofelectromagnetic frequencies ranging from UV to FIR, since these are ofincreased importance for visual appearance, thermal management, andcamouflage. However, the invention should not be limited to thesefrequencies, as similar arguments can be made for other electromagneticfrequencies.

Thermal management of predominantly enclosed spaces, includingbuildings, can be a problem in geographic locations with relatively highlevels of solar radiation, specifically in conjunction with high energycost, which makes in some countries the use of permanentair-conditioning financially problematic for a significant segment ofthe population.

A considerable portion of the energy, which contributes to indoortemperatures, is the result of absorbed solar radiation by the buildingenvelope. This effect can be true in tropical and subtropical regions,but it also applies in moderate climate zones, for example, duringsummer or times of increased relative temperature (e.g., in NorthAmerica, Central and Southern Europe).

Related problems apply also to predominantly open spaces, such as largecities and densely populated areas in tropical and subtropical regions,where concrete and asphalt surfaces can significantly contribute to thecommon “heat island” effect in such areas.

Thus, reducing the amount of solar radiation, which is absorbed by thesurface elements of predominantly enclosed spaces, including buildings,can contribute to reducing indoor temperature and/or reduce the requiredpower for air-conditioning. Furthermore, widespread application of suchadvanced passive cooling methods can contribute to mitigate the heatisland phenomena observed in large cities.

In some embodiments the disclosed invention can serve to fabricatematerials and surfaces, which enable to control to a very high degreeindependently the spectral behavior in the VIS range and NIR range,i.e., optical color and NIR “color”.

In other embodiments of the disclosed invention, such as insidesolar-thermal collectors, the exact opposite is the case: Here thetarget is to maximize the wavelength range and efficiency of conversionsof solar radiation into thermal energy.

In yet other embodiments the disclosed invention, a goal is to achieve aclose spectral match of surfaces against a given background, i.e. to“blend in” with the environment, preferable over a wide spectral range.Such effects are desirable in defense-related applications, for examplesurfaces of buildings (bunkers) or combat vehicles, to avoid, delay, orcomplicate optical, NIR, IR, or FIR (i.e., thermal) detection, includingby ground-, air-, or space-based long-range multi-spectral imagingsystems.

SUMMARY OF THE INVENTION

For purposes of summarizing the invention and the advantages achievedover the prior art, certain objects and advantages of the invention havebeen described herein. Of course, it is to be understood that notnecessarily all such objects or advantages may be achieved in accordancewith any particular embodiment of the invention. Thus, for example,those skilled in the art will recognize that the invention may beembodied or carried out in a manner that achieves or optimizes oneadvantage or group of advantages as taught herein without necessarilyachieving other objects or advantages as may be taught or suggestedherein.

In an embodiment, a method of creating a composite material is provided.The composite material includes a plurality of populations of nano- ormicro-particles and/or of nano- or micro-cavities of predominantlydistinct size distributions, shape distributions, chemical compositions,crystal structures, and crystallinity distributions. The plurality ofpopulations are predominantly embedded in a carrier material, in suchcomposition ratios, with such typical properties per population, and insuch density relative to the carrier material, that a specific targetedbroadband spectral reflectance at least within the VIS and NIR range isapproximated, which is at least partially different from the spectralproperties of each individual population within the plurality ofpopulations and the carrier.

All of these embodiments are intended to be within the scope of theinvention herein disclosed. These and other embodiments of the presentinvention will become readily apparent to those skilled in the art fromthe following detailed description of the preferred embodiments havingreference to the attached figures, the invention not being limited toany particular preferred embodiment(s) disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

A brief description of the drawings shall serve to summarize someprinciples of the disclosed invention.

FIG. 1 illustrates the average solar spectrum, which arrives at theaverage distance between the sun and the earth (i.e., at the outer edgeof the atmosphere) as well as maximum average normal irradiance at sealevel. More than 50% of the power of the solar radiation is containedwithin the NIR region.

FIG. 2 illustrates schematically the spectral reflectance of a typicalmaterial used to coat surfaces in order to achieve a certain colorand/or sealing against the water. While clearly a certain visualappearance is achieved, the material exhibits increased spectralabsorption (decreased reflectance) in the NIR range resulting in arelatively large amount of solar radiation, which is converted tothermal energy.

FIG. 3 illustrates schematically the spectral reflectance of a typicalmaterial with somewhat improved NIR properties resulting from relativelyrudimentary approaches to address this problem. Sometimes thesematerials are partially marketed as “cool products” (tiles, paintsetc.). However, such other approaches typically utilize embeddedparticles with random or at best weakly controlled and broad sizedistribution. The result is that still a substantial portion of thesolar radiation gets converted to thermal energy.

FIG. 4 illustrates simplified and schematically the disclosed method ofcreating a composite material, which contains a plurality of populationsof nano- or micro-particles and/or of nano- or micro-cavities ofpredominantly distinct size distributions, shape distributions, chemicalcompositions, crystal structures, and crystallinity distributions, whichare predominantly embedded in a carrier material, in such compositionratios, with such typical properties per population, and in such densityrelative to the carrier material, that a targeted broadband spectralreflectance at least within the VIS and NIR range is achieved.

In particular, the composition of said plurality of populations of nano-or micro-particles and/or of nano- or micro-cavities is determined byperforming an optimization loop in a high-dimensional space comprisingat least taking portions of relative amounts c_(i) [0,1] from aplurality of reservoirs R_(i) corresponding to a plurality of saidpopulations of nano- or micro-particles, mixing said portions, andembedding said mixture of said nano- or micro-particles in a carriermaterial thereby creating a composite. An amount of said compositematerial is made with sufficiently large surface to determining itsbroadband spectral properties at least within VIS and NIR. Subsequentlyan optimization algorithm determines a composition vector _(j)c of amixture of said nano- or micro-particles taken from each reservoir R_(i)during a next iterative step j. The process is repeated until acriterion, which is a measure for the degree with which the spectrumobtained from the composite produced during the last iteration doesmatch the target spectrum, has been reached or surpassed. Subsequentlylarger quantities of said composite are produced for variousapplications.

FIG. 5 illustrates schematically the spectral reflectance of a typicalcomposite material, which can be produced with the disclosed method. Inthis particular example the composite material has a specific visual(non-white) color and a very high NIR reflectance. The result is thatmost of the solar radiation in the NIR spectrum does not get convertedto thermal energy.

FIG. 6 illustrates schematically the spectral reflectance of a typicalcomposite material, which can be produced with the disclosed method. Inthis particular example the composite material has a specific visual(non-white) color. The NIR reflectance is very high in certain regionsof the NIR range but exhibits imperfections, i.e. drops at certainnarrow wavelength rages. This may be caused by the specific availableselection and the types of populations of nano- or micro-particles,which does not allow to produce completely even broadband NIR highreflectance. However, the disclosed method was in this particularexample still able to generate a useful material with increased, ornearly perfect properties since the imperfections (reflectance drops)coincide with the spectral gaps in the natural solar NIR spectrumtypically arriving on the surface of earth. Thus, still most of thesolar radiation in the NM spectrum does not get converted to thermalenergy.

DETAILED DESCRIPTION

Solar radiation, which arrives at the surface of the earth, has apeculiar spectral distribution. This is the result of the spectraldistribution of the radiation emitted by the sun, which arrives at theouter edge of the atmosphere (in first order Black-body radiationfollowing Planck's law) and the spectrally selective distribution ofscattering and absorption within the earth' atmosphere.

The electromagnetic spectrum can conceptually be divided in Ultraviolet(UV), visible (VIS), Near Infrared (NIR) [0.7 μm-3.0 μm], Mid Infrared(MIR) [3 μm-50 μm], and Far Infrared (MIR) [50 μm to 1 mm]. The exactdefinition of the boundaries of NIR, MIR, and FIR are somewhat arbitraryand different values are used by different scientific communities. (Forexample, sometimes the NIR range is defined smaller and an additionalShort Wavelength IR SWIR is defined.) For physical reasons (thecorresponding black body temperatures) we will henceforth follow thevalues given above.

As illustrated in FIG. 1, most of the solar irradiance (radiation powerdensity) is contained in the wavelength range between 0.25 μm and 3.0μm. This Figure is based on the reference solar spectral irradiance ASTMG-173. (Additional information regarding spectral properties of solarenergy can be found, for example, at the National Renewable EnergyLaboratory (NREL) website and www.nrel.gov.) Integrated over the entirespectral range, the irradiance (power density of the radiation), whicharrives from the sun at the average distance between the sun and theearth (i.e., at the outer edge of the atmosphere) is approximately 1.36kW/m². Determined by the chemical composition and density of the air andfurthermore influenced by humidity (incl. clouds), dust, pollutionlevels, latitude, time of day, date, etc., the solar radiation isattenuated while passing through the atmosphere and the spectrum isfiltered (some bands are suppressed). This is also illustrated inFIG. 1. Under best-case conditions, the peak levels of the irradiance atsea level is about 900 W/m².

As a reference, in the southwest of the United States, the averageenergy density per day on a horizontal plate is about 5 to 6 kWh/m²/dayon a horizontal plate and >9 kWh/m²/day on a surface, which is 2-axistracking to remain normal to the incidence (Source: National SolarRadiation Data Base).

The part of the spectrum, which is visible (VIS) to humans, isapproximately within the wavelength range between 380 nm and 700 nm.While this visible range corresponds to portion of the spectrum with thehighest irradiance, it contains only approximately 45% of the solarpower, which arrives at the surface of the earth. The Ultraviolet (UV)portion contains only about <4%, but the range between 750 nm and 2.5μm, which is within the NIR range, contains about 51% of the solarirradiance. (The ratios are good approximates. More precise valuesdepend on numerous other parameters.)

In order to minimize the temperature increase of an object, which isexposed to (sun)light, ideally all of the incident power should bereflected by its surfaces. Clearly, the best performance, i.e., theleast relative amount of absorbed power (i.e., the ratio of absorbedpower to arriving radiation power), would be expected from a material ora coating, which acts as a broadband reflector, i.e. is highlyreflective in UV, visible, and IR (incl. NIR, MIR, FIR), essentially abroadband mirror or at least a diffusely reflecting broadband “white”.Achieving such superior broadband reflectivity constitutes oneembodiment of the disclosed invention. Such broadband white or“super-cool” surfaces may be particularly desirable in tropical and orsubtropical regions but also e.g. in the southwest of the United States.

In some instance, however, building codes may impose constraints onusable (visual) color of buildings, or certain colors are preferred foresthetic reasons.

Thus, in another embodiment of the disclosed invention, materials arecreated whereby one is able to control, predominantly independently, thereflectivity of a surface in both the VIS and NIR parts of the spectrum,thus one can at least partially independently control the opticalappearance and the IR “appearance”.

Most conventional materials, such as those used for buildings, as wellas regular paints, have considerable NIR absorption as illustrated inFIG. 2. However, as pointed out above, at least half of the solar poweris contained in the NIR spectrum. Thus, optimizing the reflectivity inthe NIR region has the potential to reduce the absorbed solar power toat least 50%, even for surfaces, which (as the most extreme) visuallyappear to be black.

Relatively rudimentary approaches to address this problem have beenreported and are partially marketed as “cool products” (tiles, paintsetc.). However, such earlier approaches typically utilize embeddedparticles with a random size distribution or at best a weakly controlledand broad distribution.

Usually powders of various metal oxides (e.g. TiO₂, YMnO₃, Al₂O₃) orsemiconductor oxides such as SiO₂ are used, but the size distributionsof the particles is not or not well known, or varies depending on thefabrication batch, or the location from which they were collected, andis thus in general not well suited for the indented purpose. Theconsequence is, that some moderate improvements in the NIR reflectancecan be achieved, but the typical results are far from optimal, reachingreflectance values of only around 0.4, but often even considerably less.

Thus, the disclosed invention enables substantial additionalimprovements in achieving desired spectral properties of material, e.g.high NIR reflectance, if the size distribution of the embedded nano-and/or microparticles is intentionally tailored.

In the most general sense the “color” of a surface, i.e., its broadbandspectral reflection and absorption, is the result of its chemicalcomposition and, if present, of the size of contained particles and/orcavities, if they are within a certain size ratio compared to thewavelength.

More precisely, the spectral properties depend on the electronicstructure (energy levels and energy bands of electrons, including anyband gaps) as well the microscopic (on the scale of the wavelength)distribution of the refractive index n. The refractive index can bederived from the material's macroscopic relative permittivity ε_(r), andits relative permeability μ_(r) according to n=(ε_(r)·μ_(r))^(0.5). Atoptical frequencies most naturally occurring materials have a μ_(r) veryclose to 1, thus the refractive index is proximately n≈ε_(r) ^(0.5).Relative permittivity is a measure for electric polarizability, again asa result of specific electronic configurations. Thus, ultimately, n canalso be reduced to variations in the distribution of the electrondensity and polarizability. (Which is why embedded cavities can haveeffect on the spectral properties.)

Naturally occurring or synthesized chemicals, i.e. molecules, which areused as dies, have such electronic structures (energy levels), whichcause highly specific selective absorption of optical wavelength, andthus produce the distinct optical appearance (color).

However, the precise theoretical and/or numerical prediction of spectralproperties (at any wavelength range) of arbitrary materials {i.e. anynumber of chemicals, in arbitrary abundance ratios, and with arbitraryparticle size (distribution) and shape (distribution), level ofcrystallinity, potentially embedded in any other combination ofmaterials} is extremely difficult.

As the most basic case, already more than 100 years ago special solutionto Maxwell's equations obtained by Gustav Mie enabled to estimate the(elastic) electromagnetic scattering behavior of small gold particles(as reasonable approximation of so-called perfectly conducting PECspheres) in solution (water), under the assumptions that (a) theparticles are spherical and (b) sufficiently sparsely distributed toconsider their scattering behavior independent of each other. (GustavMie, Contributions to the optics of diffuse media, specificallycolloidal metallic suspensions, Annalen der Physik, 4^(th) series, Vol.25 (1908) No. 3, pp. 377-445.) Likewise, Lorenz' theory provided anapproximation of the electromagnetic wave scattering behavior ofsparsely distributed dielectric spheres.

However, both assumptions are not valid for composite materials, whichare of practical use as building materials, i.e. have potentially impacton the thermal management of contained spaces. Thus, more advancedmodels also need consider non-spherical particles or cavities (which arepractically the norm), and require the computation of thewavelength-dependent anisotropic scattering behavior of such objects.Moreover, most practical solid materials, or suspensions of particles,which are cast or applied as a liquid or gel and then dried and form asolid, must have high particle densities, which in turn necessitates toconsider cascades of multiple scattering among such particles as well asinterference effects depending on the material thickness. Realistically,such calculations need to be performed as numerical simulations forsufficient accuracy, but this creates very high computational demands(e.g. at least in part based on Finite-Difference Time Domain solversfor Maxwell's equations).

While such theoretical and numerical methods can provide very helpfulguidance for the development of materials with certain spectralqualities, in some cases experimental methods remain to be essential.One such insight is that the overall spectral properties of the materialcan be far better controlled, if the material consists of a tailoredcombination of discrete constituents, which contain nano- and/or microparticles of highly distinct size, shape, and electronic configurations.

In few cases will one be able to find natural occurring material, whichwill have spectral properties that are ideal for a consideredapplication. Thus, by using a tailored mix of nano- and/or microparticles, far better matching of the desired spectral properties of thefinal material to a target spectrum can be achieved.

Yet again, a forward determination of the optimal mix of nano- and/ormicro particles to match a given target spectrum is in general difficultif not impossible to obtain. Thus, as illustrated highly simplified andschematically in FIG. 4, the disclosed invention comprises at least thefollowing steps:

-   -   a) Preparing a plurality k of reservoirs R_(i) with i=[1, k],        each of which contains nano- or micro-particles of distinct size        distribution, shape distribution, chemical composition, crystal        structure, and crystallinity distribution. Preferably, these        distribution functions are predominantly comprised of a single        narrow peak, although multiple peaks may also be acceptable. Of        increased importance is consistency.    -    This desired simplicity of the distribution functions (per        reservoir) does not necessarily imply that their spectral        properties are simple, but it ensures their consistency, i.e. in        a sense the purity of the distributions, which is what enables        reliable and repeatable superposition of their spectral        properties to produce the targeted spectral properties of the        final composite material. This is important since due to the        high density of the various nano- or micro-particles the        electromagnetic scattering behavior depends on the presence of        other particles in the neighborhood of a given particle (i.e.,        it is not a completely orthogonal superposition).    -    In some embodiments the nano- or micro-particles for a specific        reservoir are directly found in nature but typically have to        undergo a rigorous quality assessment (i.e., experimental        determination of said distribution functions, chemical        composition, and crystallinity) to assure that said distribution        functions are maintained.    -    In some embodiments said nano- or micro-particles for a        specific reservoir are selectively filtered from a source of        less refined materials, which typically have varying and broad        distributions. Again, quality assessment is performed in        preferred embodiments in order to assure that said distribution        functions are maintained.    -    In some embodiments said nano- or micro-particles for a        specific reservoir are created by one or more chemical        processes, which tend to predominantly produce nano- or        micro-particles with distinct size distribution, shape        distribution, chemical composition, and crystallinity        distribution.    -    In some embodiments said nano- or micro-particles for a        specific reservoir are obtained by selectively filtering        particles created by one or more chemical processes, which tend        to predominantly produce particles with a wide and or varying        size distribution, shape distribution, chemical composition, and        crystallinity distribution.    -    In some embodiments said nano- or micro-particles for said        various reservoirs come from a combination of said sources.    -    In some embodiments said nano- or micro-particles from some of        said reservoirs will have similar, or even identical chemical        composition and shape distribution functions, but may vary in        terms of size distribution functions among said subset of        reservoirs. For example, there may be reservoirs with particles        with typical average sizes of 25 nm, 50 nm, 100 nm, 200 nm and        so on.    -    In some embodiments said nano- or micro-particles from some of        said reservoirs will have similar, or even identical chemical        composition and size distribution functions, but may vary in        terms of shape distribution functions among said subset of        reservoirs.    -    In some embodiments said nano- or micro-particles from some of        said reservoirs will have similar, or even identical shape        distribution functions and size distribution functions, but may        vary in terms of chemical composition or crystallinity among        said subset of reservoirs.    -    In some embodiments the nano- or micro-particles for a specific        reservoir typically have to undergo a rigorous quality        assessment (i.e., an experimental determination of said        distribution functions, chemical composition, and crystallinity)        to assure that said distribution functions are maintained.    -    Corresponding extensions of this principle should now be        apparent to those skilled in the art. The goal is to establish        preferably wide-spread points in the k-dimensional parameter        space, which is conceptually formed by the distinct particle        properties of the individual reservoirs. By way of analogy, the        reservoirs represent a broadband “color” palette, although, as        mentioned, in general a simple linear additive “color” mixing,        i.e. spectral mixing, is not possible.    -   b) Creating a mixture of said nano- or micro-particle based on        portions c_(i) taken from each reservoir R_(i). Thus, c_(i)        shall denote the relative contribution [0,1] of particles from        each reservoir to the final mixture. In many embodiments this        may be a relative contribution by mass or by volume. The entire        ensemble of all c_(i) shall subsequently be referred to as the        composition vector c.    -    In some embodiments of the disclosed invention the process of        taking the correct amount from each reservoir and mixing these        distinct populations of said nano- or micro-particle        distributions is performed by suitable, electronically        controlled equipment.    -   c) Mixing, and/or sintering, and/or dispensing and embedding        said mixture of said nano- or micro-particle in a chosen carrier        material thereby forming what shall be referred to as composite        or composite material. In many embodiments the carrier material        may initially be liquid or gel-like but will subsequently        solidify, for example by evaporation or by an induced chemical        process.    -   d) Forming a solid from said composite, or    -    creating an inhomogeneous solid, a portion of which near its        surface predominantly consists of said composite,    -    or applying a sufficiently thick amount of said composite to        the surface of another typically predominantly solid material,        and    -    the resulting area where the composite is exposed being large        enough to conduct the measurements described in the next step.    -    In some embodiments the predominantly dry particles from the        different reservoirs (under step c) only mixed and this mixture        is then applied to an at least partially wet bulk material,        which subsequently dries at ambient temperature (although this        process may create some heat), or is baked, and which thus        permanently incorporates said particles at least into its        surface region. In some such embodiments to this bulk material        may be concrete, clay, porcelain, or other ceramics. In some        embodiments some or all of the nano- or micro-particles in the        individual reservoir are in a suspension prior to mixing.    -   e) Determining the broadband spectral properties of the        resulting surface. In some embodiments this will comprise        -   Exposing the surface to light of predominantly singular            wavelength, e.g. by using a light source of predominantly            singular wavelength (e.g. a tunable laser, VCEL(s), LED(s)),            and/or by using an at least partially broadband light source            (including but not limited to arc lamps, incandescent lamps,            gas discharge lamps, globars or similar Planck radiators,            plasmas, natural solar radiation) and a variable filter,        -   Measurement of the reflectance and/or absorption of the            surface,        -   Scanning the wavelength across the wavelength range of            interest    -    (at least comprising VIS and NIR), and thus        -   producing the entire reflectance and/or absorption spectrum.    -    In some embodiments this will comprise        -   Exposing the surface to broadband EM wave spectrum with            spectral composition (at least comprising VIS and NIR) from            sources including but not limited to arc lamps, incandescent            lamps, gas discharge lamps, globars or similar Planck            radiators, plasmas, natural solar radiation,        -   Measurement of the reflectance and/or absorption of the            surface with a sensor which can discriminate wavelength or            photon energy, and        -   thus producing the entire reflectance and/or absorption            spectrum.    -    In both cases additional steps, such as those concerning        calibration, comparison with reflected spectra obtained from        reference surfaces etc., may be implemented during this step to        ensure sufficient accuracy of the obtained spectral data.    -    In some embodiments the radiation, which is reflected from the        produced surface, is measured using a sensor which is positioned        within a hollow, highly reflective hemisphere (with certain        known spectral characteristics) or a similar configuration, in        order to capture both direct and diffuse reflected radiation.    -   f) Determining the deviation of the thereby obtained reflection        or absorption spectrum from a desired target spectrum. In some        embodiments this may only be a derived single scalar, such as a        root mean square error (RMSE), i.e. the square root of the        average squared deltas at each spectral point, which is then        provided to the subsequent optimization algorithm.    -    In some embodiments that quantity may be derived by also using        weighting factor for certain spectral regions in order to        enhance or suppress the influence of said regions on the        subsequent optimization process. In some embodiment the entire        “error spectrum” function may be provided to the subsequent        optimization algorithm.    -   g) Having an optimization algorithm determine the composition        vector _(j+1)c of a mixture of said nano- or micro-particles        based on portions _(j+1)c_(i) taken from each reservoir R_(i)        during a next iterative step j+1.    -    The optimization algorithm attempts to find a point in the        k-dimensional parameter space, which corresponds to a        composition of said mixture of said nano- or micro-particles,        which exhibits spectral properties that increasingly better        match the desired target spectrum. In some embodiments the        optimization strategy may be based on a gradient method, a Monte        Carlo method, a genetic optimization algorithms, or other        suitable methods, or any combination thereof.    -   h) Going back to step b) and repeating all subsequent steps        until the RMSE value (or a similar quantity, which is a measure        for the degree with which the spectrum obtained from the        material produced during the last iteration does match the        target spectrum) has been reduced below a predetermined        threshold.    -   i) In general, there may be multiple 0-dimensional points or        lower dimensional sub-spaces (“islands”) within the        k-dimensional parameter space, which correspond to different        composition vectors c but produce spectra which are similarly        well fitting to a given target spectra. Depending on the used        optimization algorithm(s), in particular regarding any contained        true or pseudo random information sources, repeatedly conducting        the described optimization loop may result in similar, or even        identical final vectors c or may produce a plurality of        different vectors c, which still satisfy the chosen condition of        optimality, i.e. the RMSE value or a similar quantity has been        reduced below a predetermined threshold.

It shall be mentioned that from a theoretical standpoint the need forusing an optimization algorithm arises because determining a suitablecomposition vector c to produce a material with a sufficiently closematch to the desired spectrum is essentially a nonlinear inverseproblem, which can in general not be solved directly.

In some embodiments the optimization process may be aided bycomputational modeling, which attempts to predict the spectralperformance of the material produced during the subsequent iteration. Insome embodiments it may be sufficient to only rely on such computationmodels, or at least during some of the optimization steps.

Once the composition vector c for the specific set of reservoirs R (ormore specifically the set of particle properties) and the specifictarget spectrum is determined, the material can be produced in largerquantities.

There are several typical embodiments, which correspond to typicaltarget spectra:

In some embodiments the target is to produce a material with a broadbandhighly reflective spectrum (e.g., reflectivity close to 1), i.e., abroadband bright white, at least within the VIS and NIR range, or evenbeyond that. Composite materials with such properties and structuralelements made thereof are ideal for unconditionally reducing the amountof absorbed solar radiation, thus reducing the amount converted tothermal energy.

In some such embodiments the target spectral property is extended evenbeyond the NIR range into the FIR range, which in some such embodimentsis targeted to be as black as possible, i.e. the FIR reflectance is asclose as possible to zero. Such a composite material will not onlyminimize the absorption of solar radiation and its conversion to thermalenergy, but also exhibit improved performance in terms of radiatinggained heat, i.e., cooling off once exposure to direct radiation hasstopped.

In some embodiments the target is to produce a composite material with abroadband highly absorbing spectrum, i.e. a broadband black material, atleast within the VIS and NIR range. Such a composite material willconvert most of the incident radiation into heat. Application mayinclude the inside walls of solar-thermal collectors. In some suchapplications, in addition to being predominantly VIS and NIR black, thematerial will also be predominantly black in the FIR, which will causeto exhibit increased radiative thermal losses. In some other suchapplications, in addition to being predominantly VIS and NIR black, thecomposite material will also be predominantly white in the FIR, whichwill cause to exhibit reduced radiative thermal losses, i.e. it willstay warm longer after the exposure to radiation stops or is reducedbelow a level sufficient to retain its temperature.

In some embodiments the target is to produce a composite material of aspecific visual color (i.e. not white), but nevertheless with very highbroadband NIR reflectivity, as close as possible to unity. The disclosedinvention enables to fabricate such “super-cool” yet visually coloredcomposite materials with far better NIR performance compared to existingsolutions. Such materials are suitable to give buildings a desiredoptical appearance while strongly reducing the amount of absorbed solarradiation, thus reducing the amount of retained thermal energy comparedto existing solutions.

In some such embodiments the target is to produce a material of aspecific visual color (i.e. not white), with relatively high broadbandNIR reflectivity, as well as with relatively low reflectivity in the FIRrange. Such material can be made to match a desired visual color, stillexhibit relatively decreased heat gain under exposure to solarradiation, and will also comparatively rapidly cool off once exposure tosolar radiation seizes.

In some embodiments of the disclosed invention the targeted spectralproperties of the material are optimized for exposure to electromagneticradiation of comparable wavelength but other than solar radiation. Suchapplications may relate to walls of reaction chambers, turbines, ovens,furnaces, or reactors. In many such applications it is desirable toreduce excessive heating of surfaces and materials.

There are several additional embodiments in terms of the types of usednano- and micro particles although this is not meant to imply anylimitations thereon:

In some embodiments, for example, if the target is to producepredominantly broadband highly reflective materials, some of said nano-and micro-particles may consist of barium sulfate BaSO₄. In some suchembodiments the BaSO₄ particles may be derived from finely ground andsize-filtered Baryte. In other such preferred embodiments some of saidnano- and micro-particles may consist of Kaolinite—Al₂Si₂O₅(OH)₄, and/orzinc sulfide—ZnS, and/or Diopside—MgCa[Si₂O₆], and/orGoethite—α-Fe³⁺O(OH), and/or Rutile—TiO₂ with body-centered tetragonalunit cell, and/or Chrysotile—Mg₃(Si₂O₅)(OH)₄, and/or tin dioxide SnO₂,and/or Calcite—trigonal CaCO₃, and/or hydrated magnesiumsilicate—monoclinic or triclinic Mg₃Si₄O₁₀(OH)₂, and/or calcium sulfatedihydrate—CaSO₄.2H₂O, and/or titanium dioxide TiO₂, and/or aluminumoxide Al₂O₃, and/or silicon dioxide SiO₂, and/or some sheet silicateminerals including but not limited to Muscovite—KAl₂(AlSi₃O₁₀)(F,OH)₂,and/or other particles derived from similar minerals or synthesized into form particles with comparable properties.

There are several additional embodiments in terms of the used embeddingmaterial although this is not meant to imply any limitations thereon:

In some embodiments discrete populations of nano- and micro particlemixtures obtained as disclosed above are embedded in carrier materialwhich contains or predominantly contains polytetrafluoroethylene (PTFE)or other fluoropolymer.

In some embodiments discrete populations of nano- and micro particlemixtures obtained as disclosed are embedded in carrier material, whichcontains or predominantly contains polyamide-imide (incl. based ontrimellitic anhydride (TMA) and methylene dianiline (MDA)),polyethersulfone (PES), polyether ether ketone (PEEK),polymethyl-methacrylate (PMMA) or other acrylics, or polyurethanes.

In some embodiments discrete populations of nano- and micro particlemixtures obtained as disclosed are embedded in carrier material, whichcontains or predominantly contains minerals, and/or clay, and/or cement,cementitious material, or consists predominantly of concrete.

There are several additional embodiments in terms of the fabricatedstructures, which may be produced according to the disclosed invention:

In some embodiments the disclosed material is part of fabricatedstructural elements. In some such embodiments concrete, cement,cementitious material, clay, or similar minerals is cast into forms,which have said mixture of particles applied to the wall prior to thecasting process, such that said particle mixtures become predominantlyembedded to the surface regions of the finished concrete, clay, orceramic elements. This enables to create surface properties which areresistant to damage, scratching, and aging.

In some embodiments concrete, cement, clay, or other ceramics are castinto forms and said mixtures of particles are applied to the stillpredominantly wet and exposed surfaces. In some of these embodimentssaid concrete or clay elements are used to erect houses or otherbuildings and/or are parts to prefabricated houses. Such houses willhave enhanced thermal properties, e.g. offer improved living conditions(in terms of inside temperatures) without or with reducedair-conditioning.

In some embodiments the produced composite materials are used to producepredominantly sheets, plates, tiles, etc., which are mounted on thesurface of house, including roofs, and are effectively acting as solarradiation shields.

In some embodiments the disclosed composite material is fabricated as aliquid or gel-like layer with sufficiently strong adhesive propertiesand which may be applied on already erected buildings or on raw buildingmaterials.

The application of the foregoing applies to ceramic, bitumen, asphalt,tarmac, clay, mortar, cement, cementitious material, or concrete tiles,blocks, panels, roof elements, columns, girders, arcs, bricks, sheets,layers, or other elements suitable to erect structures or to be mountedonto or applied to the surface of such structures, including but notlimited to for housing, storage, protection, bridges, pavements,runways, or other means to support traffic, or other purposes. In someembodiments discrete populations of nano- and micro particle mixturesobtained as disclosed are embedded in carrier predominantly made ofasphalt.

In some embodiments, for example, in case of roofs or roof elements, theproduced composite material has arbitrary absorbance in the visiblespectrum (400 nm to 700 nm) to choose the visible color, highreflectance in the NIR range to reduce the absorption of solar energy,and high absorbance and in the mid IR and FIR, which enables improvedthermal emittance. This FIR property support the relatively rapid lossof heat, e.g. during cloudless the nights.

In some embodiments, for example, in case of walls, the producedcomposite material has arbitrary absorbance in the visible spectrum (400nm to 700 nm) to choose the visible color, high reflectance in the NIRrange to reduce the absorption of solar energy, as well as highreflectance in the MIR and FIR. Walls are typically exposed to mid andfar IR (thermal) radiation predominantly diffusely emitted by surroundsoil, vegetation, or other building and thus this targeted spectralproperty reduces heat gain also in this spectral range.

The disclosed method enables in some such embodiments, such as thoserelated to buildings, composite materials, which exhibit what may bereferred to as “super-cool” properties due to the much-improvedapproximation of ideal NM and FIR properties compared to existingapproaches and materials.

In some embodiments the disclosed composite material is applied ontopredominantly metallic elements. In some embodiments composite materialswith discrete populations of nano- and micro particle mixtures obtainedas disclosed above, are applied to the surface of vehicles, includingautomobiles, busses, or trucks, in order to reduce heat gain whileexposed to solar radiation.

In some embodiments composite materials with discrete populations ofnano- and micro particle mixtures obtained as disclosed above areapplied to the surface of satellites or space craft to reduce heat gainwhile exposed to solar radiation.

In some embodiment, such as defense-related applications, a goal is notto enhance or reduce heat gain but rather to achieve an improvedspectral match of vehicles, buildings, or other structures with thesurrounding environment. For example, many military and defense relatedstructures, such as concrete bunkers, garages, or silos etc. arefrequently painted with various camouflage patterns and in colorsmatching the direct environment, in order to make visual detection, bothby humans or by suitable detection systems operating in the VIS range,more complicated. However, such concrete structures will still havedistinct NIR reflectance spectra (NIR “color”) based on their specificcomposition, which is in general considerably different from surroundingsoil or rocks. Thus, detection by aircraft, drone, or satellite basedbroadband or NIR (“multispectral”) long-range imaging systems isrelatively easy, since the erected structures provide in general a clearcontrast in said wavelength range.

The disclosed method allows to produce composite materials, from whichsuch structures can be made, or which can be applied to or mounted ontothe surface of such structures, which spectrally more closely resembletheir environment also in the NIR and potentially FIR range. This candelay and complicate said reconnaissance and detection of saidstructures from aircraft or satellites. In addition, such properties canalso result in a better matched thermal behavior.

In some such embodiments a custom-made layer of materials producedaccording to the disclosed method is produced in the field. In suchcases the VIS and NIR spectral properties can be measured directly onthe site or in the close vicinity where a bunker or similar structure isbuilt, a material containing said tailored mixtures of nano- and/ormicro particles is produced, for example on a vehicle equipped with therequired instruments and equipment for this purpose, and applied to thestructure. In some embodiments the material may be applied as arelatively thick spray coating. In other embodiments sheets may locallybe produced, which are then mounted onto the surface of said structure.

In yet some other related embodiments spectral measurements are taken atvarious typical sites, where it is expected that structures with suchenhanced detection avoidance capabilities may be needed, and thenelements for buildings are pre-produced using sets of compositematerials, which are spectrally relatively closely matched as disclosedto said sites. In some such embodiments these may be elementspredominantly made from concrete. In some other such embodiments theseprefabricated sets of composite materials may be claylike materials,which can be applied to concrete structures, which have previously beenerected.

In some embodiments composite materials with tailored discretepopulations of nano- and micro particle mixtures obtained as disclosedabove, are applied to the surface of vehicles, including tanks, troopcarriers, trucks, or other combat vehicles in order to achieve animproved spectral matching both in VIS and NIR relative to anenvironment wherein said vehicles operate, thus complicating and/ordelaying ground-, air-, or space-based detection (including homingsystems missiles). In some such embodiments the surface of said vehiclesis effectively permanently prepared to have certain broadband spectralproperties. In some other such embodiments the surface of said vehiclesis predominantly temporarily prepared for a relatively short amount oftime, typically on the order of hours or days, but well matched to thebroadband spectral properties of the environment, which is expected tobe subsequently encountered, e.g. based on the terrain, which is to betraversed. In some such embodiments previous ground-, air-, orspace-based reconnaissance missions have at least in part obtained thedesired target spectra by measuring typical NIR spectra of the terrain,which is to be traversed.

Although the invention has been disclosed in the context of certainembodiments and examples, it will be understood by those skilled in theart that the invention extends beyond the specifically disclosedembodiments to other alternative embodiments and/or uses and obviousmodifications and equivalents thereof. Accordingly, the invention is notintended to be limited by the specific disclosures or preferredembodiments herein.

What is claimed is:
 1. A method of creating a composite material,comprising: providing a plurality of distinct populations of nano- ormicro-particles and/or of nano- or micro-cavities, a first population ofsaid plurality of distinct populations comprising one or more ofdistinct size distributions, shape distributions, chemical compositions,crystal structures, and crystallinity distributions, relative to asecond population of said plurality of distinct populations; forming acomposite material by embedding a mixture of the first population andthe second population in a carrier material, wherein forming comprisesselecting a composition ratio of the first population to the secondpopulation, and selecting the density of the mixture relative to thecarrier material, such that a specific targeted broadband spectralreflectance at least within the VIS and NIR range is approximated as aresult of interference effects between at least some of the nano- ormicro-particles and/or nano- or micro-cavities of the plurality ofdistinct populations, wherein the specific targeted broadband spectralreflectance is different from that of each individual population withinthe plurality of populations and the carrier.
 2. The method according toclaim 1, further comprising: providing a plurality of reservoirs R_(i),with i defined as an integer ranging from 1 to k, corresponding to saidplurality of said populations of nano- or micro-particles and/or ofnano- or micro-cavities; determining the composition ratios of saidplurality of populations of nano- or micro-particles and/or of nano- ormicro-cavities by performing an experimental optimization loopcomprising at least the following steps: A. creating a mixture of saidpopulations of nano- or micro-particles and/or nano- or micro-cavitiesbased on taking portions c_(i) from each reservoir R_(i), wherein c_(i)shall denote the relative contribution of nano- or micro-particlesand/or nano- or micro-cavities from each corresponding reservoir R_(i)to the mixture, thus all scalars c_(i) forming a composition vectordenoted c, B. embedding said mixture of said nano- or micro-particlesand/or nano- or micro-cavities in the carrier material thereby creatingthe composite material, C. using an amount of said composite material todetermine a broadband spectral reflectance of the composite material atleast within VIS and NIR, and D. determining a quantity, which is ameasure for the degree with which the broadband spectral reflectance ofthe composite material produced during step B does match the specifictargeted broadband spectral reflectance, E. determining if apredetermined threshold of said quantity has been reached, wherein thepredetermined threshold measures the degree with which the broadbandspectral reflectance of the composite material produced during step Bdoes match the specific targeted broadband spectral reflectance, and:E.1 if said threshold is not reached, having an optimization algorithmdetermine a new composition vector _(j+1)c of the mixture of said nano-or micro-particles and/or nano- or micro-cavities based on portions_(j+1)c_(i) taken from each reservoir R_(i) during a next iterative stepj+1 and continuing the iteration of the optimization loop at step A; andE.2 if said threshold is reached or surpassed, producing largerquantities of said composite material for subsequent use.
 3. The methodaccording to claim 1, further comprising: determining the composition ofsaid plurality of populations of nano- or micro-particles and/or ofnano- or micro-cavities by performing an optimization loop which isbased on computational modeling wherein the spectral performance of thecomposite material at least within the VIS and NIR range is determinedby computations, and wherein after said composition has beencomputationally determined, said composites are produced in largerquantities for subsequent use.
 4. The method according to any one ofclaims 1-3, wherein providing comprises distributing said plurality ofpopulations of nano- or micro-particles and/or of nano- ormicro-cavities throughout the carrier material with spatially constantdensity.
 5. The method according to any one of claims 1-3, whereinproviding comprises distributing said plurality of populations of nano-or micro-particles and/or of nano- or micro-cavities throughout thecarrier material with higher density in regions close to the surface ofthe composite material compared to the inner regions of the compositematerial.
 6. The method according to claim 5, further comprisingapplying said plurality of populations of nano- or micro-particles tosurface areas of the carrier material while the carrier material isliquid or gel-like and thereby achieving an embedding of said particles,and wherein the carrier material subsequently solidifies, thus embeddingsaid particles.
 7. The method according to any one of claims 1-3,wherein providing comprises distributing said plurality of populationsof nano- or micro-particles and/or of nano- or micro-cavities throughouta liquid or gel-like carrier material, applying the thereby formedliquid or gel-like composite material onto the surface of a solidmaterial, having the thereby formed layer subsequently solidify, andapplying said composite with at least such thickness that opaqueness ofsaid layer is achieved at least within the VIS and NIR range.
 8. Themethod according to any one of claims 1-3, with a broadband highlyreflective spectrum within the VIS and NIR range.
 9. The methodaccording to any one of claims 1-3, with a broadband highly reflectivespectrum within the UV, VIS and NIR range.
 10. The method according toany one of claims 1-3, with a broadband highly reflective spectrumwithin the VIS and NIR range, and a highly absorbing spectrum in theMIR.
 11. The method according to any one of claims 1-3, with a broadbandhighly reflective spectrum at least within the VIS and NIR range, and ahighly absorbing spectrum in the MIR and FIR.
 12. The method accordingto any one of claims 1-3, further comprising choosing the specifictargeted broadband spectral reflectance with a specific chosenreflective spectrum within the VIS range and a broadband highlyreflective spectrum in the NIR range.
 13. The method according to anyone of claims 1-3, further comprising choosing the specific targetedbroadband spectral reflectance with a specific chosen reflectivespectrum within the VIS range and a broadband highly reflective spectrumin the UV and NIR range.
 14. The method according to any one of claims1-3, further comprising choosing the specific targeted broadbandspectral reflectance with a specific chosen reflective spectrum withinthe VIS range and a broadband highly reflective spectrum in the NIRrange and a broadband highly absorbing spectrum in the MIR range.
 15. Amethod of using a composite material produced according to claim 8 onthe surface of a open structure to reduce the amount of incidentelectromagnetic radiation which is converted to thermal energy.
 16. Amethod of using a composite material produced according to claim 9 onthe surface of a open structure to reduce the amount of incidentelectromagnetic radiation which is converted to thermal energy.
 17. Amethod of using a composite material produced according to claim 8 onthe surface of a closed structure to reduce the amount of incidentelectromagnetic radiation which is converted to thermal energy and thusheat gain inside said structure.
 18. A method of using a compositematerial produced according to claim 8 on the surface of a building toreduce the amount of solar radiation which is converted to thermalenergy and thus heat gain inside said building.
 19. The method accordingto claim 8, wherein the carrier material contains minerals, and/or clay,and/or cement, cementitious material, and/or concrete.
 20. The methodaccording to claim 8, wherein the carrier material contains polymers.21. A method of using a composite material produced according to claim 9on the surface of a closed structure to reduce the amount of incidentelectromagnetic radiation which is converted to thermal energy and thusheat gain inside said structure.
 22. A method of using a compositematerial produced according to claim 9 on the surface of a building toreduce the amount of incident solar radiation which is converted tothermal energy and thus heat gain inside the building.
 23. The methodaccording to claim 9, wherein the carrier material predominantlycontains minerals, and/or clay, and/or cement, cementitious material,and/or concrete.
 24. The method according to claim 9, wherein thecarrier material contains polymers.
 25. A method of using a compositematerial produced according to claim 10 on the surface of a closedstructure to reduce the amount of incident electromagnetic radiationwhich is converted to thermal energy and thus heat gain inside saidstructure, and to also enhance thermal emittance.
 26. A method of usinga composite material produced according to claim 12 on the surface of aclosed structure to give it a certain visual color and to reduce theamount of solar radiation which is converted to thermal energy and thusheat gain inside said structure.
 27. A method of using a compositematerial produced according to claim 12 on the surface of a building togive it a certain visual color and to reduce the amount of solarradiation which is converted to thermal energy and thus heat gain insidethe building.
 28. A method of using a composite material producedaccording claim 14 on the surface of a closed structure to give it acertain visual color, and to reduce the amount of electromagneticradiation which is converted to thermal energy and thus heat gain insidesaid structure, and to enhance thermal emittance.
 29. A method of usinga composite material produced according to claim 14 on the surface of abuilding to give it a certain visual color, and to reduce the amount ofsolar radiation which is converted to thermal energy and thus heat gaininside the building, and to enhance thermal emittance and thus thermalenergy loss at night.
 30. A method of using a composite materialproduced according to any one of claims 1-3 on the surface of astructure thereby achieving a closer spectral match with the environmentat least within the VIS and NIR range.
 31. A method of using a compositematerial produced according to any one of claims 1-3 on the surface of abuilding or a vehicle thereby achieving a closer spectral match with theenvironment, and thereby reducing detectability at least within the NIRrange.